Thyroid Anatomy: Complete Guide with Parts, Functions & Diagram

Overview of Thyroid Anatomy

The thyroid gland is a small, butterfly-shaped organ in the front of the neck. It sits in the middle, just over the windpipe (trachea), and stretches from the C5 to T1 vertebrae in the spine. This gland makes important hormones—thyroid hormones (T3 and T4) and calcitonin. These helps control how the body uses energy (metabolism), support growth, and maintain calcium levels in the blood. The thyroid anatomy has two parts called lobes, one on each side of the neck, connected by a thin piece called the isthmus. The isthmus lies over the 2nd and 3rd rings of the trachea. The gland is located behind the sternothyroid and sternohyoid muscles and below the voice box (larynx). On the sides, it is close to muscles and blood vessels, and at the back, it is near the carotid artery and parathyroid glands.

Understanding the thyroid anatomy is essential for diagnosing thyroid disorders and for safely performing surgeries in the neck region.

Anatomy of the Thyroid Gland

Anatomy of Thyroid Gland

Lobes
Capsule
Blood Supply
- Arteries
- Veins

Nerve Supply
Microscopic (Histological) Anatomy
- Follicles & Follicular cells
- Parafollicular cells (C cells)

Thyroid Gland Anatomy: Parts & Functions

Lobes

The thyroid gland is a butterfly-shaped gland located in the front of the neck. It has two main parts, called the right and left lobes, which are connected in the middle by a small part called the isthmus.

Sometimes, a small extra part called the pyramidal lobe grows upward from the isthmus. A thin band of tissue, called the levator glandulae thyroideae, may connect the isthmus to a bone in the neck called the hyoid bone.

Each lobe is approximately 5 cm long, 2.5 cm wide, and 2.5 cm thick. The whole gland weighs around 25 grams. It extends from the fifth cervical (neck) vertebra to the first thoracic (upper chest) vertebra.

The lobes go from the middle of the thyroid cartilage (Adam’s apple area) down to the fifth tracheal ring (windpipe). The isthmus is about 1.2 cm in size and lies over the second and third tracheal rings.

In females, the thyroid gland can become slightly larger during menstruation and pregnancy due to hormonal changes.

Each lobe has a cone-like shape, characterized by a top (apex), a bottom (base), three surfaces (lateral, medial, and posterolateral), and two borders (anterior and posterior). The isthmus has a front and back surface, as well as top and bottom borders.

Position of the Thyroid:

Front side: Covered by skin, fascia (connective tissue), and a thin muscle called the platysma.
Backside: Close to the thyroid cartilage and the front part of the trachea.
Side: Near important blood vessels like the external carotid artery and internal jugular vein.

The thyroid gland produces hormones that regulate the body’s metabolism, energy utilization, and growth.

Capsule

The thyroid gland is enclosed by two distinct layers. The inner layer, called the true capsule, is made of strong, flexible connective tissue. Outside this lies the false capsule, which is formed from the pre-tracheal part of the deep cervical fascia.

Just beneath the true capsule is a dense network of small blood vessels known as the deep capillary plexus. During thyroid surgery (thyroidectomy), this vascular layer must be carefully removed along with the true capsule to prevent bleeding and ensure safe gland removal.

Blood Supply

Arteries

The thyroid gland receives a strong blood flow through two main arteries: the superior thyroid artery and the inferior thyroid artery, which supply its upper and lower regions, respectively.

The superior thyroid artery usually branches off first from the external carotid artery, near the top edge of the thyroid cartilage. It travels downward, behind the sternothyroid muscle, and reaches the top part of the thyroid. Along the way, it gives rise to several smaller arteries:

The infrahyoid branch (supplies muscles below the hyoid bone),
The sternocleidomastoid branch (supplies the neck muscle),
The superior laryngeal branch, which runs with the internal laryngeal nerve through the thyrohyoid membrane,
And the cricothyroid branch, which connects with its opposite-side counterpart across the midline.

The inferior thyroid artery originates from the thyrocervical trunk, which is a branch of the subclavian artery. It starts near the edge of the anterior scalene muscle, moves inward, and supplies the back part of the thyroid gland, typically reaching it near the lower third of its outer surface.

A notable branch of this artery is the ascending cervical artery, which should not be confused with the main trunk during surgery due to its similar appearance.

In some individuals (around 10%), an extra artery known as the thyroid ima artery is present. Its origin varies widely—it may arise from the brachiocephalic trunk, aortic arch, common carotid, or other nearby arteries.

Most often, it begins at the brachiocephalic trunk and supplies blood to the isthmus and the front portion of the thyroid gland.

Veins

Superior thyroid vein
Middle thyroid vein
Inferior thyroid vein

Nerve Supply

The thyroid gland lies near two important nerves that come from the vagus nerve: the external branch of the superior laryngeal nerve and the recurrent laryngeal nerve.

These nerves play a key role in voice and breathing. During thyroid surgery (thyroidectomy), there is a risk of injuring them.

If the external branch of the superior laryngeal nerve is damaged, the cricothyroid muscle on that side may become weak or paralyzed. This muscle helps control pitch, so the person may find it hard to change voice pitch or may notice their voice becomes weaker.

Injury to the recurrent laryngeal nerve on one side can prevent the vocal cord on that side from opening properly. This often causes hoarseness and may lead to breathing problems.

If both recurrent laryngeal nerves are injured, the airway can become blocked, making breathing very difficult. In such cases, a tracheostomy (a surgical opening in the windpipe) may be needed to help the person breathe.

Microscopic (Histological) Anatomy

Follicles & Follicular Cells

Each thyroid follicle is surrounded by a thin layer of connective tissue that contains fenestrated capillaries, lymphatic vessels, and sympathetic nerve fibers. These structures support the follicle by supplying nutrients, removing waste, and regulating function.

The follicular epithelium is made up of a simple layer of cells, and its shape changes depending on the follicle’s activity. When active, the cells appear cuboidal to low columnar; when inactive, they become squamous (flat).

These follicular (or principal) cells absorb iodine and amino acids from the blood through their basolateral surface. They use these materials to produce thyroglobulin, a protein rich in iodine that serves as a precursor to thyroid hormones.

This thyroglobulin is stored in the follicle’s central cavity, called the lumen, as a jelly-like substance known as colloid.

In Hematoxylin and Eosin (H&E) stained slides, the colloid stains pink, while follicular cells appear purple. The follicle’s activity can be judged by how the colloid looks:

Inactive follicles have large lumens filled with dense colloid.
Active follicles have smaller lumens and less colloid, indicating active hormone release.

Parafollicular Cells (C cells)

Scattered between or around these follicles are parafollicular cells, also called C cells. These cells are part of the neuroendocrine system (specifically, the APUD group – amine precursor uptake and decarboxylation system).

They appear pale or clear under H&E staining and are usually located within the follicular wall but do not project into the lumen. Sometimes, they are also found in small clusters between follicles.

C cells produce calcitonin (thyrocalcitonin), a hormone that helps lower blood calcium levels. It does this by reducing bone resorption (the process of breaking down bone to release calcium) and limiting calcium reabsorption in the kidneys.

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Thyroid Anatomy: Complete Guide with Parts, Names, Functions & Diagram